Being on the set of a Richard Curtis film is very like being in a Richard Curtis film. Everyone is good-looking and brisk and witty, here in the borrowed London townhouse where the 60-year-old director is shooting a short sequel to his movie Love Actually. Outside in the real world people are angry, at odds, ever more polarised. On Curtis’s closed set, a dungareed world of Lillies and Berties and Cols and Ems, trays of brownies circulate and the chat is about who slept with who once but stayed friends. Hugh Grant is present, roaming around and given licence to be caustic and urbane: “If anyone needs me I’ll be in my lair.” Otherwise the prevailing spirit is level-headedness and sympathy. “Richard likes it,” an assistant says to me, “when people are nice to each other. Plum?”

Curtis is making this short followup to Love Actually in aid of Comic Relief and Red Nose Day, causes he co-founded in the 1980s. Many of the actors from the original have agreed to return, including Grant, Colin Firth, Keira Knightley, Bill Nighy and Liam Neeson; charity tempting them back, after 14 years, to Curtisland, that preposterous and seductive fantasia-Britain that was established in a trilogy of famous romcoms: Four Weddings and a Funeral (1994), Notting Hill (1999) and Love Actually (2003). Of the three it was the last, a multi-narrative soup of soppy vignettes, written and directed by Curtis, that went on to have the most prodigious afterlife. Love Actually is now broadcast on TV with metronomic, Bond-movie regularity. In a single week last winter more than 1m copies of the film were sold on DVD in the US. At around the same time, in the UK, Love Actually was voted by the Radio Times “the nation’s favourite Christmas movie”.

On the set of the sequel, Firth ponders the movie’s substantial modern viewership and positions it in the culture somewhere alongside The Sound of Music. Grant, when asked to account for Love Actually’s enduring popularity, assumes his role as resident cynic and grumbles: “It’s unaccountable.”

Grant sips water and tries to catch his breath. He’s just shimmied his way around a bit of the set made up to look like 10 Downing Street, a grand marble staircase behind him hung with photographs of former prime ministers. His photo is among those on the wall, the actor today reprising his role as the Blair-ish PM who in the original film put aside duties of state to woo his secretary, played by Martine McCutcheon. Such is the power of love in a Richard Curtis film that Grant had to dance out his romantic vigour by wiggling up and down the halls of Downing Street to a Girls Aloud song. There’s another dance in the sequel. As with most of the new scenes in Curtis’s followup, an incident or encounter from the first film is referenced, with some sort of twist catching us up on what has happened to the character, a decade and a half on.

Incredibly, in the idealised Curtisland of 2017, Grant’s fluent, moderate prime minister is still in power. It’s a bit of unashamed wishful thinking that’s heightened, on set, by the fact that the stairway photographs of real-life prime ministers include Tony Blair and Gordon Brown, but stop short of David Cameron and Theresa May, as if these premierships (and world financial collapse and austerity and referendums and dissolution) had never come to pass. Curtis crouches under the photos, murmuring direction into Grant’s ear before they do another take.

I think I know what’s going on here, I tell the director, when he steps back behind the cameras.

“Oh yes?” Curtis says.

This is your lament, I suggest, for a time of comparative innocence. That silly, sweet, everyone-gets-along Britain of Four Weddings and Notting Hill and Love Actually – if such a place ever existed – has gone. And you are dipping back to say goodbye to it.

Curtis tilts his head. He is grey-haired but boyish, still, with big, sinless blue eyes. “I’m not sure I agree with your thesis,” he says. While the world is chaotic and uneasy right now, “there are problems that I’m hoping will not be dominant for long.” But has the old fellow-feeling really gone away? Curtis talks about Comic Relief and how it continues to be well supported by the public to do good. “Actually, I think the idea is to hold one’s nerve. Because even though things do seem to be in a very chaotic moment, people fundamentally, person-to-person, when faced for the need to be compassionate, are very compassionate.”

Curtis has Grant say something like this when they shoot a press conference scene, next. The fictional PM is asked to give the public his view on the future of world affairs, and with implausible but seductive simplicity (that patented Curtis mix) Grant insists: “Good’s going to win, I’m actually sure of it.” They do the line a few times before an assistant yells for a cut and everybody breaks for lunch. Lamb wraps.

There was a scene in Notting Hill, which also starred Grant, during which his character went along to a movie shoot: he got placed on the sidelines with a pair of headphones so that he could listen in on the actors’ dialogue. In this way he accidentally heard them mutter bitchy secrets to each other. It happens to me in eerie replication one day. A scene is being reset and a prominent actor fills the waiting with wicked chat. There is a dig at an absent castmate. Then the actor ranks by merit some of the films they have made since the last Love Actually, the one they had to be talked into, the “pay-day”. Blushing, absolutely fascinated, I listen for as long as decency and personal ethics allow before turning the volume down.

So there is a little darkness in Curtisland, after all. Commentators on the outside would suggest it has always been there. When Love Actually celebrated a 10th anniversary in 2013, various critical reappraisals appeared. Persuasive cases were made about the movie’s shaky treatment of women, especially. By this point I’d seen Love Actually enough times to have a demented familiarity with its dozen plots (Liam Neeson advising his stepson how to win the girl of his dreams, Alan Rickman cheating on his wife Emma Thompson, Andrew Lincoln declaring himself to the unattainable Keira Knightley by showing up at her doorstep with handwritten signs) but I was never able to watch it through with the same old naivety after reading Lindy West’s furious and brilliant essay for Jezebel. West pointed out how much of the plot depended on women falling in love with their male employers, suggested “Hostile Work Environment: The Movie” as an alternative title, and went as far as likening the romance between Colin Firth’s character and his Portuguese maid, played by Lucia Moniz, to sex trafficking.

The argument didn’t carry me along quite that far. But I did start to wonder what on earth would have become of these characters, after the credits rolled. Firth’s guy, having learned Portuguese to propose to his maid, would presumably have then had his first conversation in a shared language with his fiancee. Was this a sound basis for a relationship?

Some of the cast and crew had been pondering such things too. “I always assumed,” Grant tells me, “that my character was embroiled in a horrible sex scandal. And after a few years in prison he hit the bottle pretty hard. Yes – Martine abandoned him. She turned out to be very shallow.”

Curtis’s partner, Emma Freud, the script editor on the original Love Actually and the president of Comic Relief, had been wondering about the prospects of Rickman and Thompson’s marriage. Not long ago Freud and Curtis went to a late-night screening of Love Actually in New York. “While we were sitting there we asked each other: ‘Do you think they got divorced in the end?’ It was a new conversation for us.” Freud came to the conclusion, shared on Twitter, that Rickman and Thompson’s characters “stay together but home isn’t as happy as it once was”.

Freud is the social-media-facing half of this couple. Throughout the sequel shoot she roves about (fringed, charming, a super-conductor of the good feeling Curtis likes to encourage on set) with a smartphone, taking pictures, leaking titbits, deftly stirring up Twitter interest in the project. In a corner of the Downing Street set, I say to her: “You use Twitter. You must have read some of those fault-finding articles about Love Actually.”

Freud says: “Andrew Lincoln being a stalker, that sort of thing?”

Other things have been written too, I say. For instance, that it’s only the men in the film who do any chasing. The women mostly wait to be got. And about a third of them draw a pay cheque from the blokes they end up with.

Freud nods. She wasn’t aware, she says, that Love Actually had been analysed so deeply. “I’m glad to know that. I absolutely accept those criticisms. But I think that Richard when he wrote it ... He had 10 stories he wanted to tell. And I doubt the idea that those stories represented a ‘school of thought’ was in his head.” As for Freud: “There were always storylines in there that I was more fond of than others. And as time has gone on, I’ve become more fond of the ones I’m fond of. Less fond of the ones I’m less fond of.” She elects not to say which.

Grant’s work is in the can. With a promise that next time Curtis asks him to do anything for charity he’ll simply donate – “I’ll pay my way out, even if has to be a fucking big cheque” – the actor flees London and goes skiing. Firth finishes his scenes with Moniz, Neeson with his stepson. Nighy and Atkinson return to film skits. One evening, after a day in the real world during which the US president has signed another travel ban and there have been hours of debate in Westminster about Brexit, Curtis and his crew gather on a residential road in London. Curtisland is to be refashioned, one final time, for a new scene between Lincoln and Knightley.

Knightley is costumed in pyjamas and put in a doorway. Lincoln, on the doorstep, is handed a stack of felt-tipped cardboard signs. Curtis stands nearby, describing for me his dilemma when it came to writing a new storyline for this pair. The director knew he wanted to get Lincoln back outside Knightley’s house. But how to do that without it seeming “illogical, deeply creepy”? Curtis painted himself into a bit of a corner in the original, he admits, by having Lincoln’s character promise Knightley that if he couldn’t have her he would devote himself to the hopeless pursuit of Kate Moss or Helena Christensen or some other distant beauty.

I tell Curtis my theory, about what would have happened to Lincoln’s character next. There would have been the failed pursuit of Moss and the other supermodels. Court orders, of course. Then evolving resentment. That inventive romantic streak warping into something more sinister. If Lincoln was going to show up at Knightley’s door again after 14 years, I suggest, his handheld sign would surely say something like “You’ve driven me to this” or “The house is wired with explosives”. Curtis says he always had in mind something rather more innocent. Maybe Lincoln’s character could bring his kids along this time? Each with little signs of their own? He only abandoned that idea, he says, when he hit on a better one.

On set the crew hush. Cameras roll. Knightley opens her door to be met by Lincoln outside with his new pile of signs. He holds one up that reads “Meet my wife!” – at which point a woman joins him on the doorstep. She carries a sign of her own: “Hi!”

Guess who?

The surprise gets a delighted response from the crew, who chuckle, sigh, draw their duffle coats around themselves, show lovely teeth, pass around trays of lasagne. It’s a wrap. Curtis walks about insisting everybody move indoors to get warm. He asks what I think and I tell him I still believe double homicide would have been a more likely outcome. But I’m glad Curtis is more positive than I am; that he’s around to keep insisting, still, on a course of outrageous optimism. The director beams.

Red Nose Day Actually can be seen on Red Nose Day on BBC1 on Friday 24 March and NBC on 25 May